I like the question, because it's different and unique and special to everyone for different reasons.
HToC was it for me. One saturday morning, my best friend invited me and when I asked if I could bring an alt she said "Ummm... I don't think it's the kind of place you can box"
I remember going in, determined, coming out 5 hours later to do dailies because I couldn't afford the repair bill anymore, and 4 hours after that I said in guild chat, "People who say it can't be done will go out and prove themselves right.", just as the paladin stood alone over the Black Knight's body.
Nowadays, things like Garfrost etc, while they make me rage, are nothing compared to what I went through in ToC.